The invention relates to a high pressure metal vapor discharge lamp having
an outer envelope, PA1 a lamp stem sealing said outer envelope in a gas-tight manner and including a pair of stem conductors extending within said outer envelope in a common plane, PA1 a pair of arc tubes each having a ceramic wall enclosing a discharge space, pair of metallic feed-throughs extending from each end thereof, and a discharge sustaining fill in said discharge space, and PA1 a metallic lamp frame for supporting said arc tubes within said outer envelope and electrically connecting said arc tubes in parallel, said frame being electrically connected to each of said metallic arc tube feed-throughs. PA1 the lamp frame supports the arc tubes in a plane parallel with the stem conductors, is welded directly to each of the feed-throughs of the pair of arc tubes and only in a plane co-planar with the stem conductors, and is free of slip-fit connections with the feed-throughs.
Such a lamp is known, for example, from U.S. Pat. Nos. 4,868,499 and 4,906,888 (Dunn et al) and U.S. Pat. No. 4,287,454 (Feuersanger et al) which disclose a dual arc tube high pressure sodium discharge lamp. In each of these lamps the lamp frame supports the arc tubes longitudinally within the envelope and aligned with each other in a plane transverse to the plane extending through the stem conductors.
Because of the electrically parallel configuration, a discharge is maintained in only one of the arc tubes during lamp operation. If the one arc tube is extinguished during operation, for example due to an interruption in the power supply, the other arc tube will ignite first upon restoration of power to the lamp because of its lower temperature. The arc tube which was previously operating will remain off. Thus, during operation the arc tubes will have different lengths due to thermal expansion because one of the arc tubes will be at a higher temperature. Additionally, their lengths will change upon switching the lamp on and off.
In these lamps, the frame includes a member which is welded directly to the metallic feed-throughs at one end of the arc tubes. At the other end of the arc tubes, the frame includes a stabilizing member which includes a portion inserted into each of the tubular feed-throughs with a slip fit. The slip fit allows for changes in length of the ceramic arc tubes independently of each other due to the above-mentioned thermal expansion. To ensure a good electrical connection with the feed-throughs which have the slip-fit with the stabilizing member, additional flexible conductors in the form of straps or wires are welded to the stabilizing member and these feed-throughs.
The frames for the above lamps are rather complex and include numerous metallic frame parts which must be accurately positioned and welded together. Since the frame components extend in three dimensions, the welds which connect the frame to the feed-throughs are spaced in a plane which is transverse to the welds which connect the frame to the stem conductors. Frame components that extend in three dimensions typically require more handling steps or complex automation equipment to complete all welds. Accordingly, it is difficult to automate their assembly and the welding of the individual frame components to each other.